1. Field of the Invention
The present invention relates to a lead frame, more particularly to a lead frame having a silver-plated layer formed on its surface.
2. Description of the Prior Arts
Lead frame is an important component in fabrication processes of semiconductor devices and light-emitting diodes. In addition to the function of supporting the die formed thereon, the lead frame also acts as a circuit board to transmit the inner functions of the electronic components to the outside parts.
With reference to FIG. 9, a conventional lead frame comprises a metal base 71 and a silver-plated layer 72. In electronic packaging, a die 81 is mounted onto the silver-plated layer 72 of the lead frame and electrically connected with the silver-plated layer 72 by multiple bonding wires 82, so as to transmit electronic signals. In the final stage of fabrication, a molding compound 91 is employed to encapsulate the lead frame, the die 81, and the bonding wires 82 to protect them from physical or chemical damages, thereby achieving the packaging and protection of the integrated circuit.
As silver has super conductivity, the silver-plated layer 72 is formed on the metal base 71 for increasing the signal transmission rate. However, the adhesive strength between the silver-plated layer 72 and the molding compound 91 is insufficient. Thus, the molding compound 91 is apt to delaminate from the lead frame after moisture absorption occurs.
To pass a more severe moisture sensitivity level, a metal surface treatment provided in the prior art is utilized. The metal base of the lead frame is surface-roughened by etching, and a silver-plated layer deposited on the roughened metal base is surface-roughened accordingly. As a result, the roughness treatment can increase the contact area of the roughened silver-plated layer with the molding compound and improve the adhesive strength between the lead frame and the molding compound.
Although the metal surface treatment can try to avoid the occurrence of delamination of the molding compound from the lead frame, the roughened surfaces reduce the bonding forces of wires welded onto the roughened metal base or the roughened silver-plated layer, such that the wire disconnection and the open circuit failure between the die and external circuit likely happen to the conventional lead frame due to the insufficient wire bonding force.
To overcome the shortcomings, the present invention provides a lead frame to mitigate or obviate the aforementioned problems.